Transform-domain feedback signaling for MIMO communication

ABSTRACT

The present invention relates to a control method for a communication network that has a transmitter with an array of transmit antennas and that has at least one receiver communicating with the transmitter. The receiver performs a channel measurement for a receive antenna of the receiver using a signal transmitted from the transmitter to the receiver. The receiver further determines channel coefficients for each of an array of transmit antennas at the transmitter from an output of the channel measurement, and then applies a linear, reversible and orthogonal transform to the channel coefficients, thus ascertaining channel component coefficients indicative of the individual weight of respective channel components in a transform domain. The receiver then selects one or more channel components in the transform domain and communicates to the transmitter a control signal indicative of one or more preferred channel components or a magnitude of one or more channel component coefficients, or both, in quantized form. The transmitter receives the control signal and constructs a beam pattern in the transform domain using the information received from the receiver.

FIELD OF THE INVENTION

The present invention relates to:

-   -   a control method for a communication network that has a        transmitter with an array of transmit antennas and that has at        least one receiver communicating with the transmitter;    -   a receiver module for a communication network with a transmitter        having an array of transmit antennas;    -   a control-signaling method for a receiver in a communication        network with a transmitter having an array of transmit antennas        for transmission to at least one receiver;    -   a computer program product, containing executable program code,        which is configured to cause a receiver to carry out a        control-signaling method for a receiver in a communication        network that has a transmitter with an array of transmit        antennas for transmission at least to one receiver;    -   a receiver for a communication network with a transmitter having        an array of transmit antennas;    -   a transmitter control module for a transmitter having an array        of transmit antennas for transmission to at least one receiver;    -   a transmitter having an array of transmit antennas for        transmission to at least one receiver;    -   a transmitter control method for a transmitter having an array        of transmit antennas for transmission to at least one receiver;    -   a computer program product, containing executable program code,        which is configured to cause a transmitter having an array of        transmit antennas for transmission to at least one receiver to        carry out a transmitter control method;    -   a communication network, comprising a transmitter having an        array of transmit antennas for transmission to at least one        receiver; and    -   a control signal for a communication network that has a        transmitter with an array of transmit antennas and that has at        least one receiver communicating with the transmitter.

BACKGROUND OF THE INVENTION

Multiple-input multiple-output (MIMO) is a technology for nextgeneration wireless systems to enhance the capacity and robustness ofthe communication link. MIMO technology is based on the presence ofmultiple transmit antennas and multiple receive antennas in thecommunication link. Application of MIMO technology is envisioned forcellular communication, broadband wireless access, as well as forwireless local area networks (WLANs). A plurality of two or moretransmit antennas is also referred to as an array of transmit antennasherein.

The benefits of MIMO communication are obtained through a combination ofantenna arrays that provide spatial diversity from the propagationchannel and algorithms that can adapt to the changing multivariatechannel.

In future mobile systems and in the long-term evolution of the UniversalMobile Telecommunication System (UMTS LTE) the use of multiple-antennatechniques will become increasingly important to meet spectralefficiency requirements. A significant gain in spectral efficiency canbe achieved in a downlink transmission by multiplexing multiplecodewords in the spatial domain to either a single user or multipleusers sharing the same time-frequency resource block. These single-useror multi-user MIMO schemes exploiting the multiplexing gain ofmulti-antenna transmission are sometimes referred to as spatial divisionmultiplexing (SDM) and spatial division multiple access (SDMA)techniques. An SDMA scheme enables multiple users within the same radiocell to be accommodated on the same frequency or time slot. Therealization of this technique can be accomplished by using an antennaarray, which is capable of modifying its time, frequency, and spatialresponse by means of the amplitude and phase weighting and an internalfeedback control.

Beamforming is a method used to create a radiation pattern of theantenna array by constructively adding the phases of the signals in thedirection of the communication targets (terminal devices) desired, andnulling the pattern of the communication targets that are undesired orinterfering.

In this context, the beamforming vector plays an important role. Forpurposes of illustration of the meaning of the beamforming vector, in anexemplary single-user communication system employing transmitbeamforming and receive combining, assuming that signaling is done usingM transmit and N receive antennas, the input-output relationship of thiscommunication system is given by:y=z ^(H) Hwx+z ^(H) nwhere H is a N×M channel matrix connecting the transmitter and thereceiver, z is the receive combining vector, z^(H) is its Hermitiantranspose, w is the transmit beamforming vector, x is the transmittedsymbol from a chosen constellation, and n is independent noise added atthe receiver.

One of the challenges in the design of the beamforming vectors for SDMand SDMA techniques is the need for the base station to know thechannels for all the users and receiving antennas of each user. Thiswould require a large amount of feedback to be signaled from the usersto the base station.

Solutions have been proposed to reduce this signaling information byintroducing a codebook of few possible beamforming matrices. Each userthen applies a greedy procedure to select one or more preferredbeamforming vectors out of the codebook, by evaluating theSignal-to-Noise-Ratios (SINRs) of different beamforming combinations.Thus, each user has to signal one or several indexes of the preferredvector or vectors, respectively, plus one or moreChannel-Quality-Indicator (CQI) values, indicating the correspondingSINRs.

An issue with codebook-based solutions is that the beamforming vectorsare not jointly optimized according to the channel conditions. The basestation uses the feedback information from the users only to scheduletransmission to the set of users reporting the best CQI values.

Alternatively, significant gain in the cell throughput can be achievedif the base station could implement an ad-hoc design of the beamformer.This is possible, for example, if the users report all the channelcoefficients, after some quantization operation. However, this requiressignaling as many complex values as the product, MN, between the numberM of transmit antennas and the number N of receive antennas per user.

SUMMARY OF THE INVENTION

According to a first aspect of the invention a control method for acommunication network that has a transmitter with an array of transmitantennas and that has at least one receiver communicating with thetransmitter is provided. The method comprises the steps:

-   A) at the receiver:    -   performing a channel measurement for a receive antenna of the        receiver using a signal transmitted from the transmitter to the        receiver;    -   determining the channel coefficients for each of an array of        transmit antennas at the transmitter from an output of the        channel measurement;    -   applying a reversible transform to the channel coefficients,        thus ascertaining channel component coefficients indicative of        the individual weight of respective channel components in a        transform domain;    -   selecting one or more channel components in the transform        domain;    -   communicating to the transmitter a control signal, which is        indicative of either:

1) one or more preferred channel components, derived from the lineardecomposition of the vector of channel measurements, and a magnitude ofone or more channel components, or, alternatively,

2) one preferred channel component and an estimate of the SINR or,alternatively,

3) one preferred channel component;

-   B) at the transmitter:    -   receiving the control signal; and    -   constructing a beam pattern in the transform domain using the        information received from the receiver.

The method of the invention allows an advantageous control of abeamforming process on the transmitter side, based on communicatingchannel component coefficients indicative of the individual weight ofrespective channel components in a transform domain from the receiver tothe transmitter.

The method overcomes a main issue with codebook-based solutions, namely,that beamforming vectors are not jointly optimized according to thechannel conditions. In codebook-based implementations, the transmittertypically uses feedback information from the receivers, like wirelessmobile terminal devices, only to schedule transmission to the set ofusers reporting the best CQI values.

Significant gain in the cell throughput can be achieved by the method ofthe invention because the transmitter can implement an ad-hoc design ofa beamformer. This is possible, because the receiver reports channelcomponent coefficients as defined by the alternatives 1) to 3) above,and preferably in quantized form. In the prior art, this would requiresignaling as many complex values as the product, MN, between the numberof transmit antennas, M, and receive antennas, N, per receiver. Inembodiments of the method of the first aspect of the invention, however,it suffices to extract some channel state information (CSI) bits fromchannel measurements at the receiver side. This information is enough toenable the transmitter, for instance a base station, to design a robustbeamforming matrix. The amount of feedback bits required is exactly thesame as in the codebook-based techniques.

Note that the terms transmitter and receiver are chosen with referenceto the role of the respective device in a communication. Generally, thetransmitter of the claims can also take the role of a receiver in adifferent communication, and the receiver of the claims can also takethe role of the transmitter in a different communication. Both devicescan be transceiver devices that are configured to perform the methodsteps associated with the transmitter and configured to perform themethod steps associated with the receiver. A preferred configuration foran application of the method of the invention is that of a base stationforming the transmitter and a mobile terminal forming the receiver in aMIMO communication network.

In the following, embodiments of the control method of the first aspectof the invention will be described. Unless explicitly described asalternatives, the embodiments can be combined with each other.

In one embodiment performing a channel measurement comprises measuringpilot information transmitted from each of the array of transmitantennas at the transmitter. Preferably, the pilot signals are designedto be orthogonal between each antenna of the array of transmit antennas.

Advantageously, the reversible transformation applied to the channelcoefficients is linear and orthogonal; this helps reduce thecomputational complexity at the receiver. In one embodiment where thereversible transformation applied to the channel coefficients is linearand orthogonal, the transform comprises an Inverse Discrete FourierTransform (IDFT). The transform domain can be angular domain. Thechannel components in the transform domain indicate different angles orangular intervals. The channel-component coefficients in this embodimentcan take the form of an ordered set of index values, indicating amagnitude associated with an angle or angular interval determined by theindex order.

Preferably, thus, the feedback information communicated from thereceiver to the transmitter may comprise one or more preferred receivingdirections determined from data in the transform domain. This feedbackinformation may be quantized as an integer indexing an angularquantization grid.

In one particular embodiment, the transform is performed in accordancewith the equation

$\begin{matrix}{{{\hat{h}}_{a,l} = {\frac{1}{\sqrt{M}}{{\sum\limits_{m = 0}^{M - 1}{{\hat{h}}_{m}{\mathbb{e}}^{j\; 2\pi\mspace{11mu}\frac{m\; l}{L}}}}}}},{l = 0},\ldots\mspace{14mu},{L - 1}} & (1)\end{matrix}$

This is to be understood as follows: Let ĥ denote an M-dimensionalvector with components ĥ_(m) of channel measurements that the receiverhas derived for a given receiving antenna, for instance by sensingcommon pilots embedded in a resource block received from thetransmitter. Furthermore, M is the number of transmit antennas at thetransmitter. It is assumed without restriction that n bits of an uplinksignaling channel are reserved for communicating the channel-componentcoefficients, for instance in the form of index values, such that anangular quantization grid provides L=2^(n) levels. The receiver or, ifmultiple receivers are present, each receiver computes an L-point IDFTof ĥ, and takes the absolute value, i.e. forms a vector ĥ_(a) withelements ĥ_(a,l) as given by equation (1). This vector ĥ_(a) representsa quantized version of the angular-domain response amplitude of thechannel.

It should be noted that the IDFT is a particular example of a suitabletransform. Other transforms could be used that are a linear, reversibleand orthogonal.

A further embodiment comprises a step of determining a measure ofuncertainty of the channel component coefficients in the transformdomain at the receiver after applying the linear and reversibleorthogonal transform to the channel coefficients. For example this couldbe determined from the transformed coefficients in the form of abeamwidth. As another example, this could be determined from the rangeof variation of sets of channel coefficients or channel componentcoefficients obtained at different times or at different frequencies.

Another embodiment comprises a step of applying a filter to the channelcomponent coefficients in the transform domain at the receiver. Thisembodiment forms one way of including the effects of uncertainty in themeasured channel coefficients. In the case of the angular domain, use ofa smoothing filter is equivalent to including angular uncertainty.

In another embodiment, selecting one or more channel components in thetransform domain comprises ascertaining and selecting a channelcomponent that has a channel component coefficient, which forms aneither absolute or relative maximum of magnitude in the set of channelcomponent coefficients. With reference to the embodiment, in which thechannel component coefficients belong to the angular domain and areindicative of a magnitude in a particular angular direction, theembodiment forms a direction finding technique. Other direction findingtechniques could be employed

For a beamforming process envisaged, which in one embodiment is based ona covariance matrix of the channel coefficients, the phase of a singlepeak is not required. In the case of more than one peak, however, thephase difference between peaks would be required in order to enable tocorrectly recover an estimate of the channel coefficients and/orcovariance matrix. A further embodiment therefore comprises:

-   -   ascertaining more than one relative maximum of magnitude in the        set of component coefficients;    -   measuring an amplitude of the respective relative maxima;    -   measuring a phase difference between the channel components        forming the relative maxima; and    -   measuring a lower-bound estimate of the        signal-to-interference-plus-noise ratio (SINR) given by the        following expression. Let ∥ĥ∥ be the Euclidean norm of the        vector of channel measurements, and ĥ_(a)*=|ĥ·ĥ_(a)*| be the        modulus of the inner product between the vector of channel        measurements and its quantized representation, i.e. the peak        amplitude of the function ĥ_(a,l). We also define P as the ratio        between the transmit power and the thermal noise power at the        receiver. Then, the estimate is given by

${SINR} = {\frac{\frac{P}{M}{\hat{h}}_{a}^{*2}}{1 + {\frac{P}{M}\left( {{\hat{h}}^{2} - {\hat{h}}_{a}^{*2}} \right)}}.}$

In the case of multiple peaks phase information would be included withthe magnitudes in the communicating step.

In one embodiment, communicating the control signal to the transmittercomprises communicating via a radio link, wherein control informationcomprised by the control signal, as defined in alternatives A), 1) to3), mentioned above, is preferably provided in quantized form.

In a further embodiment the possible quantized channel representationsare stored in a codebook, which would allow more flexibility in thedesigning the channel quantizer to match the expected channelconditions. To support convenient codebook design, the codebook may bearranged such that the entries form unitary matrices. For any codebook(unitary or not) the selection of a preferred channel representation ismade by finding the index of the codebook entry with the highest valueof estimated SINR. This corresponds to identifying a preferred codebookindex. In estimating the SINR, a range of assumptions may be made aboutthe interference from other transmissions, including the presence oftransmissions on other beams and the precoding applied to thesetransmissions. Preferably these different assumptions would correspondto different decisions at the transmitter, for example to scheduletransmissions to different users on different beams. The transmissionson other beams may use precoding selected from a codebook, which wouldhave the advantage of limiting the number of possible beamformers, whichwould have to be implemented at the transmitter and the number ofdifferent possible beams, which would need to be taken into account atthe receiver when considering assumptions about interference.

At the receiver, since the codebook index corresponding to the highestestimated SINR will in general depend on the assumed interference,different preferred codebook indices may be determined for differentinterference assumptions, In the case of a unitary codebook the indicesmay be constrained to be from the same unitary matrix (which limits thenumber of possibilities that need to be considered, leading to lowerreceiver complexity, but lower performance) or allowed to be fromdifferent unitary matrices (which is more flexible, but may lead to moresignaling overhead to cover the additional possibilities).

In general, to enable the transmitter to decide on the scheduling oftransmissions to different users and the transmission details (e.g. bitrate, coding, time/frequency domain resource allocation, use ofbeamforming/precoding), one or more of preferred codebook indices fordifferent interference assumptions and some or all of the correspondingCQI values may be communicated to the transmitter. The set ofassumptions to be considered at the receiver may be pre-determined, forexample as a part of the system design, or configurable by signaling,since there will in general be a trade-off between increasingperformance by considering a wider range of assumptions and theassociated additional complexity and signaling overheads (e.g. fromsending more indices and CQI values to the transmitter).

For multiple receive antennas receiver, the method of the first aspectof the invention can be extended by executing it once for each of thereceive antennas. In one embodiment, therefore, the channel measurementis performed for each of an array of receive antennas at the receiver.

The same procedure can be applied for virtual receive antennas formed bylinear combinations of signals from more than one physical antenna.

Preferably, the step of communicating to the transmitter comprisescommunicating a set of channel quality indicator (CQI) bits.

On the transmitter side, constructing a beam pattern in the transformdomain in one embodiment comprises constructing a transmit signalimpulse in at least one of the one or more preferred channel components,where the preferred channel component can for instance form preferredangular directions.

In this embodiment, constructing a beam pattern in the transform domainpreferably comprises constructing the transmit signal impulse with anamplitude determined according to the magnitude of the channel componentcoefficients.

Another embodiment further comprises a step of determining a measure ofuncertainty in the transform domain of the channel componentcoefficients at the transmitter after receiving the control signal.Determining a measure of uncertainty in the transform domain of thechannel component coefficients at the transmitter can for examplecomprise evaluating a range of variation of the preferred channelcomponents communicated at different times or at different frequencies.

Another embodiment of the method of the first aspect of the inventioncomprises applying a filter to the beam pattern in the transform domain.For example, this filter could be determined according to an obtainedmeasure of uncertainty.

Another embodiment comprises designing a beamformer for transmissionfrom the transmitter to the receiver in dependence on the constructedbeam pattern in the transform domain. Designing a beamformer can forinstance comprise applying an inverse of the linear and reversibleorthogonal transform, e.g., a Discrete Fourier Transform (DFT) thatforms an inverse to the IDFT mentioned earlier, to the channel componentcoefficients.

Furthermore, the beamformer design process could be based oncoefficients received from more than one receiver. Thus, designing abeamformer is in another embodiment performed in additional dependenceon channel component coefficients received from at least one secondreceiver different from the receiver.

In a further embodiment the beamformer is restricted to be one of a setof pre-coding coefficients from a pre-determined codebook. Limiting theflexibility of the pre-coding may lead to lower complexity at thetransmitter, and may lead to less signaling overhead (e.g. if thecodebook index is signaled to the receiver, rather than unrestrictedvalues of pre-coding coefficients). For convenient design, the codebookmay be constructed from unitary matrices.

In an embodiment with codebooks at both transmitter and receiver, thecodebooks may be different. This means that the contents and size ofthese codebooks can be independently designed to optimized the trade-offbetween such factors as system performance vs. complexity and signalingoverhead.

Note that beamforming techniques using the channel covariance matrix donot require the absolute phase of the channel coefficients.

Note that in some cases a channel can be assumed to be reciprocal (orapproximately reciprocal) between uplink and downlink, so measurementsmay then be made in one direction and applied in the other. In this casethe following steps would not be required:

-   -   selecting one or more channel components in the transform        domain;    -   communicating to the transmitter a control signal, which is        indicative of either:

1) one or more preferred channel components, derived from the lineardecomposition of the vector of channel measurements, and a magnitude ofone or more channel components, or, alternatively,

2) one preferred channel component and an estimate of the SINR or,alternatively,

3) one preferred channel component;

-   -   constructing a beam pattern in the transform domain using the        information received from the receiver during the communicating        step;

Note that a vector quantizer can be applied to the channel measurements,instead of performing the steps:

-   -   selecting one or more channel components in the transform        domain;    -   measuring an amplitude of respective relative maxima in the set        of channel component coefficients;    -   measuring a phase difference between the channel components        forming the relative maxima.

However, this would involve a higher process complexity, and becausephase information is not required in many cases, may involve signaling alarger number of bits.

In a MIMO communication network, a transmitting station selects abeamforming matrix based on feedback information from one or morereceiving stations. According to the invention, the feedback informationis derived from an Inverse Discrete Fourier Transform (IDFT) of ameasure of channel characteristics made on a signal received at thereceiving station.

Optionally the feedback information may comprise an indication of peakmagnitude of signals in the transform domain.

Optionally the feedback information may comprise an indication ofangular uncertainty in the transform domain.

Optionally a filter may be applied to the data in the transform domain.

Alternatively, a different orthogonal transform may be used instead ofthe IDFT.

According to a second aspect of the invention, a control signalingmethod for a receiver in a communication network with a transmitterhaving an array of transmit antennas for transmission of user data tothe receiver is provided, comprising:

-   -   performing a channel measurement for a receive antenna of the        receiver;    -   determining the channel coefficients for each of an array of        transmit antennas at the transmitter from an output of the        channel measurements;    -   applying a reversible transform to the channel coefficients,        thus ascertaining channel component coefficients indicative of        the individual weight of respective channel components in a        transform domain;    -   selecting one or more channel components in the transform        domain;    -   communicating to the transmitter a control signal indicative of        either:

1) one or more preferred channel components, derived from a lineardecomposition of a vector formed by the channel coefficients, and amagnitude of the one or more channel components, or, alternatively,

2) one preferred channel component and an estimate of a quantityindicative of a signal-to-interference-plus-noise ratio SINR, or,alternatively,

3) one preferred channel component.

Embodiments of the control signaling method of the second aspect of theinvention correspond to those above described embodiments of the methodof the first aspect of the invention, which further specify method stepsperformed at the receiver, see also claims 2 to 27.

A third aspect of the invention is a computer program product,containing executable program code, which is configured to implement acontrol signaling method for a receiver in a communication network thathas a transmitter having an array of transmit antennas for transmissionof user data to the receiver, the control signaling method comprisingthe steps:

-   -   performing a channel measurement for a receive antenna of the        receiver;    -   determining the channel coefficients for each of an array of        transmit antennas at the transmitter from an output of the        channel measurements;    -   applying a reversible transform to the channel coefficients,        thus ascertaining channel component coefficients indicative of        the individual weight of respective channel components in a        transform domain;    -   selecting one or more channel components in the transform        domain;    -   communicating to the transmitter a control signal indicative        either:

1) one or more preferred channel components, derived from a lineardecomposition of a vector formed by the channel coefficients, and amagnitude of the one or more channel components, or, alternatively,

2) one preferred channel component and an estimate of a quantityindicative of a signal-to-interference-plus-noise ratio SINR, or,alternatively,

3) one preferred channel component.

In particular, the computer program product can be a receiver controlsoftware or a firmware stored on a data medium or implemented in areceiver, or an update software for a previous control software orfirmware on a data medium.

Embodiments of the computer program product of the third aspect of theinvention contain executable program code, which is configured toimplement a control signaling method for a receiver according toembodiments of the method of the first aspect of the invention whichfurther specify method steps performed at the receiver, see also claims2 to 27.

According to a fourth aspect of the invention, a receiver module for acommunication network with a transmitter having an array of transmitantennas is provided. The receiver module comprises:

-   -   a channel-measurement unit, which is configured to be connected        with a receive antenna of a receiver and configured to perform a        measurement of a physical quantity indicative of a channel        quality for the receive antenna on the basis of the received        signal and to provide an output signal indicative of the        measurement result in the form of channel coefficients for each        of the array of transmit antennas at the transmitter;    -   a transform unit, which is connected with the        channel-measurement unit and configured to apply a linear and        reversible orthogonal transform to the channel coefficients, and        to provide at its output channel component coefficients        indicative of the individual weight of respective channel        components in a transform domain;    -   a selection unit, which is connected with the transform unit and        configured to select one or more channel components in the        transform domain on the basis of the channel component        coefficients received from the transform unit;    -   a control unit, which is configured to generate and provide at        its output a control signal indicative of either:

1) one or more preferred channel components, derived from a lineardecomposition of a vector formed by the channel coefficients, and amagnitude of the one or more channel components, or, alternatively,

2) one preferred channel component and an estimate of a quantityindicative of a signal-to-interference-plus-noise ratio SINR, or,alternatively,

3) one preferred channel component.

The receiver module can form a component of a receiver or anindependently traded add-on module to an existing receiver of a previousgeneration.

In the following, embodiments of the receiver module will be described.The embodiments can be combined with each other, unless stated otherwiseexplicitly. Further details and advantages of the embodiments can befound in the context of the description of embodiments of the method ofthe first aspect of the invention, which further specify method stepsperformed at the receiver.

In one embodiment, the channel-measurement unit is configured to performa channel measurement by measuring pilot information transmitted fromeach of the array of transmit antennas at the transmitter.

In another embodiment, the transform unit is configured to apply anInverse Discrete Fourier Transform to the channel coefficients.

In a further embodiment the channel-measurement unit is furtherconfigured to determine a measure of uncertainty of the channelcomponent coefficients in the transform domain.

In another embodiment, the channel-measurement unit is furtherconfigured to apply a filter to the channel component coefficients inthe transform domain.

In one embodiment, the control unit is configured to generate andprovide at its output an uncertainty indicator that forms a measure ofuncertainty of a channel component coefficient in the transform domain.

The selection unit is in another embodiment configured to ascertain andselect a channel component that has a channel component coefficient,which forms an either absolute or relative maximum of magnitude in thecomponent coefficients received from the transform unit.

In a further embodiment, the selection unit is configured to:

-   -   ascertain whether more than one relative maximum of magnitude is        present in the set of channel component coefficients received        from the transform unit;    -   measure an amplitude of respective relative maxima in the set of        channel component coefficients received from the transform unit;    -   measure a phase difference between the channel components        forming the relative maxima.

The control unit is in one embodiment configured to communicate to thetransmitter in quantized form one or more preferred channel componentsor a magnitude of one or more channel component coefficients via a radiolink.

In a further embodiment, the channel-measurement unit is configured to:

-   -   be connected with an array of receive antennas; and to    -   perform the channel measurement for each antenna of the array of        receive antennas.

In another embodiment, the control unit is preferably configured tocommunicate to the transmitter the control signal form in the form of aset of channel quality indicator bits.

According to a fifth aspect of the invention, a receiver for acommunication network with a transmitter having an array of transmitantennas is provided, comprising:

-   -   at least one receive antenna, which is configured to receive a        signal transmitted by the array of transmit antennas of the        transmitter;    -   a receiver module according to claim the fourth aspect of the        invention or one of its embodiments described herein.

The receiver comprises in one embodiment an array of receive antennas.

In another embodiment, that can be combined with any of the otherembodiments, the receiver forms a mobile terminal device for wirelesscommunication.

In another embodiment, the receiver also comprises a transmitteraccording a seventh aspect of the invention or one of its embodiments,which will be described below.

According to a sixth aspect of the invention, a transmitter controlmodule is provided, which is configured:

-   -   to receive from an external device a control signal indicative        either:

1) one or more preferred channel components, derived from a lineardecomposition of a vector formed by the channel coefficients, and amagnitude of the one or more channel components, or, alternatively,

2) one preferred channel component and an estimate of a quantityindicative of a signal-to-interference-plus-noise ratio SINR, or,alternatively,

3) one preferred channel component.

and

-   -   to construct and provide at its output beam-pattern control data        in the transform domain using the information received from the        external device.

The transmitter control module can form a component of a transmitter oran independently traded add-on module to an existing receiver of aprevious generation.

In the following, embodiments of the transmitter control module will bedescribed. The embodiments can be combined with each other, unlessstated otherwise explicitly. Further details and advantages of theembodiments can be found in the context of the description ofembodiments of the method of the first aspect of the invention, whichfurther specify method steps performed at the transmitter.

In one embodiment, the transmitter control module is further configuredto:

-   -   construct and provide beam-pattern control data for a beam        pattern in the form of a transmit signal impulse in at least one        of the one or more preferred channel components.

Another embodiment of the transmitter control module further comprises achannel-evaluation unit, which is configured to generate and provide atits output an uncertainty indicator forming a measure of uncertainty ofthe channel component coefficients in the transform domain.

According to a seventh aspect of the invention, a transmitter isprovided, comprising:

-   -   an array of transmit antennas; and    -   a transmitter control module according to the sixth aspect of        the invention or one of its embodiments described herein.

In one embodiment the transmitter forms a base station in amobile-communication network.

In a further embodiment, the transmitter also comprises a receiveraccording to the fifth aspect of the invention or one of itsembodiments.

According to an eighth aspect of the invention, a transmitter controlmethod is provided, comprising the steps:

-   -   receiving a control signal indicative of one or more preferred        channel components or a magnitude of one or more channel        component coefficients, or both, in quantized form;    -   constructing a beam pattern in the transform domain using        information contained in the control signal.

Embodiments of the transmitter control method correspond to thoseembodiments described in the context of the description of embodimentsof the method of the first aspect of the invention, which furtherspecify method steps performed at the transmitter.

According to a ninth aspect of the invention, a computer program productis provided, containing executable program code, which is configured toimplement a transmitter control method for a transmitter having an arrayof transmit antennas for transmission of user data to the receiver, thetransmitter control method comprising the steps:

-   -   receiving a control signal indicative of one or more preferred        channel components or a magnitude of one or more channel        component coefficients, or both, in quantized form;    -   constructing a beam pattern in the transform domain using        information contained in the control signal;

Embodiments of the computer program product of the ninth aspect of theinvention contain executable program code, which is configured toimplement a transmitter control method corresponding to thoseembodiments described in the context of the description of embodimentsof the method of the first aspect of the invention, which furtherspecify method steps performed at the transmitter, see also claims 42 to53.

In particular, the computer program product can be a transmitter controlsoftware or a firmware stored on a data medium or implemented in atransmitter, or an update software for a previous transmitter controlsoftware or firmware on a data medium.

A tenth aspect of the invention is formed by a communication network,comprising a transmitter according to the eighth aspect of the inventionor one of its embodiments and a receiver according to the fifth aspectof the invention or one of its embodiments.

The communication network is in one embodiment a cellular wirelessnetwork, preferably according to a Universal Mobile CommunicationStandard. Communication between network entities of the communicationnetwork preferably employs a Multiple-input multiple-output (MIMO)technology.

An eleventh aspect of the invention is formed by a control signal for acommunication network that has a transmitter with an array of transmitantennas and that has at least one receiver communicating with thetransmitter, the control signal comprising control informationindicative of either:

1) one or more preferred channel components, derived from a lineardecomposition of a vector formed by the channel coefficients, and amagnitude of the one or more channel components, or, alternatively,

2) one preferred channel component and an estimate of a quantityindicative of a signal-to-interference-plus-noise ratio SINR, or,alternatively,

3) one preferred channel component.

The control information can for instance be provided in quantized form.

Embodiments of the invention are also defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to thedrawings in which:

FIG. 1 a shows an 8-level angular quantization grid resulting from thecalculation of an angular response via an 8-point IDFT.

FIG. 1 b illustrates the quantization diagram.

FIGS. 2 a and 2 b show the amplitude of the angular domain response of achannel realization, normalized by the peak value, in a Cartesian andpolar plot, respectively.

FIG. 3 shows a format of a control signal used in an uplink signalingchannel.

FIGS. 4 to 9 show numerical results of simulating the system and method.

FIG. 10 shows a flow diagram of an embodiment for a control method for acommunication network that has a transmitter with an array of transmitantennas and a receiver communicating with the transmitter.

FIG. 11 shows a block diagram of an embodiment of a receiver suitablefor performing the operations of the receiver in the control method ofFIG. 10.

FIG. 12 shows a schematic block diagram of a transmitter suitable forperforming the operations of the transmitter in the control method ofFIG. 10.

DETAILED DESCRIPTION OF EMBODIMENTS

As an exemplary communication system we consider a cellular downlinktransmission towards K receivers, with a single receiving antenna each.The signal at a generic receiver can expressed as

$y_{k} = {{h_{k}{\sum\limits_{i = 1}^{K}{w_{i}x_{i}}}} + n_{k}}$where h_(k) is the M-dimensional row-vector with the time-sample channelcoefficients ‘seen’ by user k, w_(i) are the beamforming vectors whichmultiplex the data symbols x_(i) to the M transmit antennas and n_(k) isthe additive noise component. In the following we drop the subscript kdenoting a generic user.

Let us indicate with ĥ the M-dimensional vector of channel measurementsthat the receiver, which hereinafter will also be referred to as theterminal, has derived for a given receiving antenna, by sensing thecommon pilots embedded in the resource block. M is the number oftransmit antennas at the transmitter, which hereinafter will also bereferred to as the base station or, in short, BS. We assume that n bitsof the uplink signaling channel are reserved for reporting the indexvalues, such that the angular quantization grid provides L=2^(n) levels.Each terminal computes an L-point IDFT of ĥ, and take the absolutevalue, i.e. forms a vector ĥ_(a) with elements

$\begin{matrix}{{{\hat{h}}_{a,l} = {\frac{1}{\sqrt{M}}{{\sum\limits_{m = 0}^{M - 1}{{\hat{h}}_{m}{\mathbb{e}}^{j\; 2\pi\frac{ml}{L}}}}}}},{l = 0},\ldots\mspace{14mu},{L - 1}} & (1)\end{matrix}$

This vector represents a quantized version of the angular-domainresponse amplitude of the channel ĥ, given by

$\begin{matrix}{{{\hat{h}}_{a}(\phi)} = {\frac{1}{\sqrt{M}}{{\sum\limits_{m = 0}^{M - 1}{{\hat{h}}_{m}{\mathbb{e}}^{j\; 2\pi\; m\;\Delta\;{si}\; n\;\phi}}}}}} & (2)\end{matrix}$where φ is the angle formed by a given propagation direction with theboresight of the transmit array, and Δ is the transmit antennaseparation normalized by the carrier wavelength.

The terminal then reports one or more points of the function ĥ_(a,l),typically the peak index and the peak value, computed as

$\begin{matrix}{l^{\star} = {\arg\;{\max\limits_{l}{\hat{h}}_{a,l}}}} \\{{\hat{h}}_{a}^{\star} = {\hat{h}}_{a,l^{\star}}}\end{matrix}$

Alternatively, the terminal may report an estimate of the SINR, forinstance a lower bound estimate of the SINR, provided by:

${SINR} = \frac{\frac{P}{M}{\hat{h}}_{a}^{*2}}{1 + {\frac{P}{M}\left( {{\hat{h}}^{2} - {\hat{h}}_{a}^{*2}} \right)}}$

Reporting the lower-bound estimate of the SINR brings a surprisinglyhigh efficiency to the method and system of the invention, as witnessedfrom numerical simulation results. This reporting can be done forinstance with help of some of the bits of channel quality indicator bits(CQI).

This estimate can be made under a plurality of assumptions. For example,this estimate could be done under the assumption that the transmitterforms the beam pattern by using zero forcing beamforming.

In addition, the terminal may report an estimate of the angular spread,by measuring the width of the peak lobe. This can be signaled, forexample, by reporting the number of points of ĥ_(a,l) falling within 3dB attenuation at either side of the peak (see the example hereafter).

Note that the transformation for the angular-domain representation ofthe channel measurements is an IDFT because of the geometry of thetransmit array, which is assumed to be an ULA (uniform linear array).Different geometries imply different transformations from the spatial tothe angular domain. However, the terminal could still be reportingchannel measurements in terms of angular indices, under the assumptionof a transmit ULA, independently of the actual array geometry. It is,then, up to the BS to associate these indices with the correct anglesand corresponding unit spatial signatures, depending on the arraygeometry. This step may be needed for the BS to be able to steer a beamin the correct angular direction.

Note that the terminal does not need to know the antenna spacing at thetransmitter. However, this parameter is known to the BS, hence it canassociate the reported peak index with a physical propagation angle. Infact, the IDFT operation encompasses a uniform quantization of thedirectional sine, Ω=sin φ, with step size

$\frac{2}{L} = {\frac{1}{2^{n - 1}}.}$As a consequence the quantization on the angle φ is non-uniform. Anglesare more finely quantized around the boresight (φ=0) than near thedirection of the array broadside. This is a desirable property as anuniform linear array (ULA) has maximum angular resolution, approximatelyequal to

$\frac{1}{M\;\Delta}$radians, along the boresight.

The angular information is derived by the BS as follows

$\phi^{*} = {\arcsin\;\frac{l^{*} - {{{fix}\left( \frac{2\left( {l^{*} - 1} \right)}{L} \right)}L}}{L\;\Delta}}$where ‘fix’ denotes the round-towards-zero operation.

Note that if the antennas are densely spaced (Δ<1/2) the equation abovemay have no solution. However, this is a case of very little practicalinterest, as the antenna arrays are usually designed with Δ≧1/2 formaximum uncorrelation properties. Moreover, as the antenna spacingdecreases below half a wavelength, the radiation pattern becomes closeto that of an omnidirectional antenna, and the directional informationbecomes less significant.

In the following, an example will be described with reference to FIGS. 1a through 2 b in parallel.

Let us consider a 4-antenna ULA covering a 120° cell sector. Thenormalized antenna spacing is set to Δ=1/√{square root over (3)}. Thenumber of bits reserved for signaling the angular index is fixed to n=3,thus L=8.

FIG. 1 a shows an 8-level angular quantization grid resulting from thecalculation of the angular response via an 8-point IDFT. FIG. 1 billustrates the quantization diagram. Note that the directional sine isuniformly quantized. FIGS. 2 a and 2 b show the amplitude of the angulardomain response of a channel realization (normalized by the peak value)in a Cartesian and polar plot, respectively. Bullets represent thequantized version computed by eq. (1), while the solid line denotes thefunction of eq. (2). Note that in order to plot the angular response,knowledge of the antenna spacing is needed, which is necessary toassociate the IDFT indices to angular values.

FIG. 3 shows a format of a control signal S in an uplink signalingchannel as proposed herein: the first 3 bits are used to report the IDFTindex, which is 7 (111) in the example, corresponding to the peak of theangular domain response. The second field contains the value of theamplitude of the angular response at the peak. The third optional fieldcontain an indication of the angular spread: for example the terminalcan count the number of IDFT points with amplitude within 3 dB of thepeak value, at either side of the peak. In the example this number is 1(001). The BS can then derive an estimate of the angular spread of themain lobe at 3 dB, which is roughly 25° as can be seen from FIG. 2 a.

Numerical Results

We have compared the channel vector quantization (CVQ) technique andPU2RC in terms of average throughput and average number of active usersper sub-carrier use, where average is w.r.t. the ensemble ofindependently generated channel matrices H. We have considered twodifferent channel models.

Independent Rayleigh fading. The elements of H are i.i.d. properGaussian random variables ˜CN(0,1). This model generates completelyuncorrelated channels in space for each user.

3GPP spatial channel model (SCM) [13]. We report results for twoopposite scenarios.

Sub-urban macro, with a single path propagation (briefly, SCM-SM1Path).This models a very spatially-correlated channel for each user, withnearly line-of-sight propagation.

Urban micro, with 10-path propagation (briefly, SCM-Um10Path). Thissimulates a rich scattering environment with low spatial correlation.

We consider the case of M=4 transmit antennas and K=20 single-antennausers. We assume that CSI from the UE's is sent to the Node B onzero-delay, error-free feedback channels and that each UE has perfectknowledge of its channel and no knowledge of the others'. Moreover, weassume that a codebook of N M-dimensional vectors is known to both theNode B and the UE's, and that each UE feeds back a log(N) bit index andan analog (i.e. unquantized) real CQI value.

As a baseline reference to evaluate the spatial multiplexing gain of theMU-MIMO techniques, we consider a TDMA-type of system where for eachchannel instance the transmitter selects the user with the bestachievable rate. In this baseline system each UE performs channel vectorquantization and reports the quantization index and the following:

-   CQI-   CQI_(k,TDMA=∥h) _(k)∥² cos²θ_(k).    The beamforming vector, g_(k) is given by:-   g_(k,TDMA)=Pĥ_(k) ^(H),    and the user rate:-   R_(k,TDMA)=log(1+P ·CQI_(k,TDMA))    is achievable. The multiplexing gain—defined as the limit of the    ratio R/log₂ (SNR) for high SNR—of this baseline system is one,    independently of the level of CSI available at the transmitter. The    accuracy of the quantization in this TDMA system only affects the    SNR offset w.r.t. the perfect CSI curve.

For comparison, we have also plotted the dirty paper coding (DPC)sum-rate capacity curve and the achievable throughput for ZF beamformingwith water-filling power allocation across users (briefly, ZFWF), withgreedy user selection and perfect CSI available at the transmitter.

The first case (FIG. 4) is the Rayleigh fading channel model, i.e.spatially uncorrelated channels, with B=4, 8 and 12 quantization bits.For the CVQ technique and the baseline TDMA, we have used Grassmanniancodebooks G(4,1,16) and G(4,1,256) for B=4 and 8 bits respectively,while RVQ is used for B=12 bits. We have tried RVQ also for fewerquantization bits and performance is, within a fraction of dB, close tothe Grassmannian codebooks. We recall that the Grassmannian codebookshave been generated by random search amongst vectors isotropicallydistributed in the M-dimensional complex unit sphere. We can see thatPU2RC performance loses out to TDMA and CVQ for all SNR and quantizationlevels. For 8-bit quantization and upwards, the CVQ technique, witheither Grassmann codebooks or RVQ, provides the best performance in thewhole SNR range. One clear issue with a PU2RC-type of scheme is that themultiplexing gain is bounded above by one in the limit of largecodebooks. This is because, if p=1/L=M/2^(B) is the probability that auser selects a given beamforming matrix in the codebook, the probabilityof l out of K users selecting the same matrix is a binomial randomvariable with parameters (p, K), β (p, K), and mean value l=Kp. Hencethe average number of users selecting the same beamforming matrixdecreases exponentially with the number of quantization bits B.Eventually, for large B, if K is kept constant, only a single user willever be allocated. This can be clearly seen in FIG. 5 where the averagenumber of active users is plotted versus SNR. On the other hand userallocation for CVQ gradually increases with the SNR and with B up to amaximum of 4.

In FIGS. 6 and 8 the SCM channel model is evaluated with the “sub-urbanmacro” scenario and one propagation path (SCM-SMPath1 for short), andthe “urban micro” scenario, ten paths (SCM-UmPath10 for short),respectively. The SCM-SMPath1 channel models a nearly line-of-sightpropagation condition with local scattering at the receivers and veryhigh spatial correlation. The SCM-UmPath10 models a rich scatteringurban scenario. Because of the spatial correlation, i.e. “directional”properties in the angular domain, of these two channels, Grassmannianand random codebook are not well suited. In fact, these codebooks aredesigned specifically for uncorrelated channel vectors whose directionis isotropically distributed in the M-dimensional unit sphere. Thecorrelation properties of the SCM channel are better captured by aFourier codebook. Such a codebook structure is used for the CVQ schemeand TDMA in FIGS. 6-9. We note that vector quantization using suchcodebook can be done very efficiently by DFT-transform. Moreover, theentire codebook need not be stored in memory as the vector quantizationoperation boils down to a simple Fourier transform. In FIGS. 7 and 9 theaverage number of allocated users is shown for the SCM-SMPath1 andSCM-UmPath10 channel models, respectively.

FIG. 10 shows a flow diagram of an embodiment for a control method 100for a communication network that has a transmitter T with an array oftransmit antennas and a receiver R communicating with the transmitter T.

In one implementation, the transmitter T is a Node B provided with MIMOtechnology in a cellular communication network according to the UMTSstandards, and the receiver R is a mobile terminal attached to thecommunication network via the Node B. In such an implementation therewould typically be more than one mobile terminal each with a receiver,but the method is described principally by reference to one receiver inone mobile terminal.

Structural details of the transmitter T and the receiver R will bedescribed with reference to FIGS. 11 and 12. For the purpose of thepresent embodiment, only the transmitter characteristics of the basestation will be discussed. However it is understood that a Node Btypically also comprises a receiver portion.

In the flow diagram of FIG. 10, method steps performed at the receiver Rside and at the transmitter T side are shown in different branches ofthe flow diagram, labeled accordingly. The method steps performed oneach side form respective individual control methods for the transmitterand the receiver, respectively.

The method 100 is started with the transmitter sending pilot informationthrough each of the array of transmit antennas provided at thetransmitter side, in step 102. The pilot signals are orthogonal betweeneach antenna of the array of transmit antennas. The pilot signalstransmitted to the receiver are used on the receiver side to performchannel measurements for a receive antenna of the receiver, at step 104.Subsequently, at step 106, channel coefficients are determined for eachelement of the array of transmit antennas of the transmitter T from theoutput of the channel measurement.

Subsequently, the determined channel coefficients are subjected to areversible transform so as to obtain channel component coefficients,which are indicative of an individual weight of respective channelcomponents in the transform domain, at step 108. An example of asuitable reversible transform is an Inverse Discrete Fourier Transform(IDFT). However, other reversible transforms can be used. Linear andorthogonal transforms are preferred because they reduce thecomputational complexity at the receiver. The transform domain in thepresent embodiment is an angular domain such that the channel componentsin the transform domain indicate different angles or angular intervals.Thus, the channel component coefficients determined in this embodimentform an ordered set of magnitude values associated with an angle orangular interval determined by an index order.

Subsequently, at step 110, the receiver R selects one or more of thechannel components in the transform domain. In this selecting step, thereceiver ascertains and selects a channel component that has a channelcomponent coefficient, which forms either an absolute or a relativemaximum of magnitude in the set of channel component coefficients. Instep 112, the receiver R provides one or more preferred channelcomponents, which are derived from the linear decomposition of thevector of channel measurements, and a magnitude of one or more channelcomponents to the transmitter. In an alternative embodiment, thereceiver communicates to the transmitter a control signal indicative ofone preferred channel component and an estimate of asignal-to-interference-plus-noise ratio (SINR). In a further alternativeembodiment, the receiver communicates to the transmitter control signalindicative of one preferred channel component.

In a step 114, the transmitter T receives the control signal, and, atstep 116, it constructs a beam pattern in the transform domain using theinformation received from the receiver R, with which step the methodends. In an embodiment with more than one receiver R, the beam patternmay be constructed using information from more than one receiver.

FIG. 11 shows a simplified block diagram of an embodiment of a receiver200 suitable for performing the operations of the receiver in thecontrol method of FIG. 10. The receiver of FIG. 11 is shown only in thatamount of detail, which is relevant for explaining the features of thetechnology disclosed by the present application. Known functionalitiesof a receiver for operation in a MIMO communication network are notshown for the sake of simplicity and conciseness of the presentspecification.

The receiver 200 is designed for operation in the communication networkdescribed with reference to FIG. 10 and for performing acontrol-signaling method for a receiver that corresponds to the methodsteps performed by the receiver R in FIG. 10. The receiver 200 has anarray 202 of receive antennas, of which two receive antennas 204 and 206are shown. In an alternative embodiment, the receiver 200 has only onereceive antenna.

The receiver 200 has a channel measurement unit 208, which on its inputside is connected with the array of receive antennas 202. Thechannel-measurement unit 208 is on its output side connected with atransform unit 212. The transform unit 212 is connected with a selectionunit 214, which in turn is connected with a control unit 216. Thecontrol unit is connected with a transmitter portion 218, which isconnected on its output side with a transmit antenna 220. The units 208to 216 can take the form of a receiver module 222, which can be used toupdate existing receiver structures with the technology describedherein. As such, the receiver module 222 will have input and outputports 224, 226, and 228.

In operation, the channel-measurement unit 208 performs a measurement ofa channel coefficients for each of the array of transmit antennas at thetransmitter on the basis of the received signal. The received signalused by the channel measurement unit is typically a pilot signalcontaining pilot information received from a MIMO transmitter (in theexample of FIG. 10: the Node B) via an array of transmit antennas of aMIMO transmitter. The channel measurements are performed for eachantenna of the array of receive antennas 202.

Channel coefficients obtained from the channel measurements are fed tothe transform unit 212, which performs a reversible transform to thechannel coefficients, preferably a linear and orthogonal transform,which is suitably a IDFT. The transform unit provides at its outputchannel component coefficients to the selection unit 214. The channelcomponent coefficients indicate an individual weight of the respectivechannel components in the transform domain, which, as explained issuitably an angular domain. The selection unit 214 selects one or morechannel components in the transform domain on the basis of the channelcomponent coefficients received from the transform unit. The selectionis suitably based on a criterion, which allows to determine either anabsolute or a relative maximum of magnitude in the channel componentcoefficients received from the transform unit 212. The selection unitthen provides the selected channel component information to the controlunit 216. The control unit 216 forwards to the transmit portion 218 oneor more preferred channel components, as selected by the selection unit216, or a magnitude of one or more channel component coefficients, orboth. In an alternative embodiment, the control unit provides onepreferred channel component, as selected by the selection unit 216, andan estimate of the SINR. This information is processed by the transmitportion 218 for transmitting it to the transmitter (not shown in FIG.11) via the transmit antenna 220.

FIG. 12 shows a simplified block diagram of a transmitter 300 suitablefor performing the operations of the transmitter in the control methodof FIG. 10. The transmitter 300 of FIG. 12 is shown in only an amount ofdetail, which is relevant for explaining the features of the technologydisclosed by the present application. Known functionalities of atransmitter for operation in a MIMO communication network are not shownfor the sake of simplicity and conciseness of the present specification.

The transmitter 300 has an array 302 of transmit antennas and receiveantennas, which are shown as a single combined array in FIG. 12. Thetransmitter further has a transmitter control module 304, which isconfigured to receive from the receive antennas a control signalindicative of one or more preferred channel components and a magnitudeof the one or more channel components.

The control signal has been described with reference to FIGS. 3, 10 and11 in further detail. In an alternative embodiment, the received controlsignal is indicative of one preferred channel component and an estimateof the SINR.

From the received information, the transmitter-control unit 304constructs beam-pattern control data in the transform domain andprovides it to the antenna array 302 in the form of transmit controlsignals to be used during transmission of data to a receiver, from whichthe external control signal indicative of the one or more preferredchannel components were received. Those functional blocks of thetransmitter, which are responsible for generation of transmit signals tobe fed to the MIMO antenna array 302 are omitted in the block diagram ofFIG. 12.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims.

A single unit may fulfill the functions of several items recited in theclaims. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage.

A computer program may be stored/distributed on a suitable medium, suchas an optical storage medium or a solid-state medium supplied togetherwith or as part of other hardware, but may also be distributed in otherforms, such as via the Internet or other wired or wirelesstelecommunication systems.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality.

The invention claimed is:
 1. A communication method comprising:performing a channel measurement for the at least one receive antenna ofat least one receiver using a signal transmitted from at least onetransmitter to the at least one receiver; determining channelcoefficients for each of the array of transmit antennas at thetransmitter based on an output of the channel measurement; applying areversible transform directly to the determined channel coefficients toascertain channel component coefficients indicative of an individualweight of respective channel components in a transform domain, whereinthe transform domain is an angular domain, and wherein an absolute valueof a channel component is indicative of a magnitude in a particularangular direction; selecting one or more channel components in thetransform domain; communicating to the at least one transmitter acontrol signal indicative of one of: one or more preferred channelcomponents, derived from a linear decomposition of a vector formed bythe channel coefficients, and a magnitude of the one or more channelcomponents, or one preferred channel component and an estimate of aquantity indicative of a signal-to-interference-plus-noise ratio SINR,wherein the control signal comprises at least two fields, a first fieldfor reporting an inverse discrete fourier transform (IDFT) indexcorresponding to one of the selected preferred channel components and asecond field indicating the value of the magnitude of said one of thepreferred channel components or a value indicative of the SINR; andwherein the control signal is configured to be utilized to construct abeam pattern in the transform domain.
 2. The method of claim 1, whereinthe reversible transform is linear and orthogonal.
 3. The method ofclaim 1, wherein performing a channel measurement further comprisesmeasuring pilot information transmitted from each of the array oftransmit antennas at the at least one transmitter.
 4. The method ofclaim 2, wherein applying a reversible transform to the channelcoefficients comprises applying the IDFT to the channel coefficients. 5.The method of claim 1, further comprising determining a measure ofuncertainty of the channel component coefficients in the transformdomain at the at least one receiver after applying the linear andreversible orthogonal transform to the channel coefficients.
 6. Themethod of claim 5, further comprising applying a filter to the channelcomponent coefficients in the transform domain at the at least onereceiver.
 7. The method of claim 5, wherein communicating to the atleast one transmitter further comprises communicating a measure ofuncertainty in the transform domain.
 8. The method of claim 1, whereinselecting the one or more channel components in the transform domainfurther comprises ascertaining and selecting a channel component thathas a channel component coefficient, which forms one of an absolute or arelative maximum of magnitude in the set of channel componentcoefficients.
 9. The method of claim 1, further comprising ascertainingmore than one relative maximum of magnitude in the set of componentcoefficients; measuring an amplitude of the respective relative maxima;and measuring a phase difference between the channel components formingthe relative maxima.
 10. The method of claim 1, comprising communicatingto the at least one transmitter in quantized form one or more preferredchannel components or a magnitude of one or more channel componentcoefficients comprises communicating via a radio link.
 11. The method ofclaim 1, wherein the at least one receiver has an array of receiveantennas, and the channel measurement is performed for each of the arrayof receive antennas at the at least one receiver.
 12. The method ofclaim 1, wherein the at least one receiver is part of a mobile terminaldevice for wireless communication and the at least one transmitter is abase station.
 13. The method of claim 1, wherein communicating to the atleast one transmitter is in quantized form and comprises communicating aset of channel quality indicator bits.
 14. The method of claim 13,wherein at least part of the set of channel quality indicator bitsencode a signal-to-interference-plus-noise (SINR) estimate.
 15. Themethod of claim 14, wherein the SINR estimate is an approximate lowerbound SINR estimate.
 16. The method of claim 15, wherein the approximatelower bound SINR estimate is given by a ratio wherein the numeratorcomprises the amplitude corresponding to the communicated channelcomponent, and the denominator comprises a mean value of a plurality ofpoints derived from the amplitude response of the channel.
 17. Themethod of claim 16, wherein the SINR is derived as:${SINR} = \frac{\frac{P}{M}{\hat{h}}_{a}^{*2}}{1 + {\frac{P}{M}\left( {{\hat{h}}^{2} - {\hat{h}}_{a}^{*2}} \right)}}$where ĥ is an M-dimensional vector of channel measurements, M is thenumber of transmit antennas at the at least one transmitter, P is aratio between the transmit power and the thermal noise power at the atleast one receiver, and ĥ_(a) is a vector obtained by computing anL-point IDFT of ĥ and taking the absolute value.
 18. The method of claim14, wherein the SINR estimate is computed under a pre-determined set ofassumptions.
 19. The method of claim 18, wherein the set of assumptionscomprises that the beam pattern constructed by the at least onetransmitter is formed using zero forcing beam-forming.
 20. The method ofclaim 1, wherein the control signal is utilized to construct a beampattern in the transform domain at the at least one transmitter byconstructing a transmit signal impulse in at least one of the one ormore preferred channel components.
 21. The method of claim 20, whereinthe control signal is utilized to construct a beam pattern in thetransform domain by constructing the transmit signal impulse with anamplitude determined according to the magnitude of the channel componentcoefficients.
 22. The method of claim 1, further comprising applying afilter to the beam pattern in the transform domain.
 23. A receiver for acommunication network, the network including the receiver and at leastone transmitter having an array of transmit antennas, the receivercomprising: a channel calculation circuit, configured to be connectedwith a receive antenna of the receiver and configured to perform ameasurement of a physical quantity indicative of a channel quality forthe receive antenna on the basis of a received signal and to provide anoutput signal indicative of the measurement result in the form ofchannel coefficients for each of the array of transmit antennas at theat least one transmitter; a transform circuit connected with the channelcalculation circuit and configured to apply a reversible transformdirectly to the channel coefficients, and further configured to provideat its output channel component coefficients indicative of an individualweight of respective channel components in a transform domain, whereinthe transform domain is an angular domain and wherein an absolute valueof a channel component is indicative of a magnitude in a particularangular direction; a selection circuit coupled to the transform circuitconfigured to select one or more channel components in the transformdomain on the basis of the channel component coefficients received fromthe transform circuit; a control circuit configured to generate andprovide at its output a control signal indicative of one of: one or morepreferred channel components resulting from the reversible transformapplied to angular channel coefficients, the coefficients belonging tothe angular domain, wherein the absolute value of a coefficient isindicative of a magnitude in a particular angular direction and amagnitude of the one or more channel components, or one preferredchannel component resulting from the reversible transform applied toangular channel coefficients, the coefficients belonging to the angulardomain wherein the absolute value of a coefficient is indicative of amagnitude in a particular angular direction and an estimate of aquantity indicative of a signal-to-interference-plus-noise ratio SINR,wherein the control signal comprises at least two fields, a first fieldfor reporting an inverse discrete fourier transform (IDFT) indexcorresponding to one of the preferred channel components and a secondfield indicating the value of the magnitude of said one of the preferredchannel components or a value indicative of the SINR estimate.
 24. Thereceiver of claim 23, wherein the reversible transform is linear andorthogonal.
 25. The receiver of claim 23, wherein the channelcalculation circuit is configured to perform a channel measurement bymeasuring pilot information transmitted from each of the array oftransmit antennas at the transmitter.
 26. The receiver of claim 25,wherein the transform circuit is configured to apply the InverseDiscrete Fourier Transform to the channel coefficients.
 27. The receiverof claim 23, wherein the channel calculation circuit is furtherconfigured to determine a measure of uncertainty of the channelcomponent coefficients in the transform domain.
 28. The receiver ofclaim 23, wherein the channel calculation circuit is further configuredto apply a filter to the channel component coefficients in the transformdomain.
 29. The receiver of claim 23, wherein the control circuit isconfigured to generate and provide at its output an uncertaintyindicator that forms a measure of uncertainty of a channel componentcoefficient in the transform domain.
 30. The receiver of claim 23,wherein the selection circuit is configured to ascertain and select achannel component that has a channel component coefficient, which formsone of an absolute or a relative maximum of magnitude in the componentcoefficients received from the transform circuit.
 31. The receiver ofclaim 23, wherein the selection circuit is configured to: ascertainwhether more than one relative maximum of magnitude is present in theset of channel component coefficients received from the transformcircuit; measure an amplitude of respective relative maxima in the setof channel component coefficients received from the transform unit;measure a phase difference between the channel components forming therelative maxima.
 32. The receiver of claim 23, wherein the controlcircuit is configured to communicate to the at least one transmitter inquantized form one or more preferred channel components or a magnitudeof one or more channel component coefficients via a radio link.
 33. Thereceiver of claim 23, wherein the channel circuit is configured to:connect with an array of receive antennas, and perform a channelmeasurement for each antenna of the array of receive antennas.
 34. Thereceiver of claim 23, wherein the control circuit is configured tocommunicate to the at least one transmitter the control signal form inthe form of a set of channel quality indicator bits.
 35. A method for areceiver in a communication network, the network including the receiverand a transmitter having an array of transmit antennas for transmissionto the receiver, the method comprising: at the receiver: performing achannel measurement for a receive antenna of the receiver; determiningchannel coefficients for each antenna of the array of transmittertransmit antennas based on an output of the channel measurements;applying a reversible transform directly to the channel coefficients toascertain channel component coefficients indicative of an individualweight of respective channel components in a transform domain, whereinthe transform domain is an angular domain, and wherein an absolute valueof a channel component is indicative of a magnitude in a particularangular direction; selecting one or more channel components in thetransform domain; communicating to the transmitter a control signalindicative of one of: one or more preferred channel components resultingfrom the reversible transform applied to angular channel coefficients,the coefficients belonging to the angular domain wherein the absolutevalue of a coefficient is indicative of a magnitude in a particularangular direction and a magnitude of the one or more channel components,or one preferred channel component resulting from a reversible transformapplied to angular channel coefficients, the coefficients belonging tothe angular domain wherein the absolute value of a coefficient isindicative of a magnitude in a particular angular direction and anestimate of a quantity indicative of a signal-to-interference-plus-noiseratio (SINR), and wherein the control signal comprises at least twofields, a first field for reporting an inverse discrete fouriertransform (IDFT) index corresponding to a peak of the angular domainresponse and a second field indicating a value of an amplitude of theangular response at the peak or a value indicative of the SINR estimate.36. The method of claim 35, wherein the reversible transform is linearand orthogonal.
 37. A non-transitory computer-readable storage mediumthat is not a transitory propagating signal or wave, the medium havingstored thereon control information including instructions that causecomputer processing circuitry to: perform a channel measurement for areceive antenna of the receiver; determine channel coefficients for eachof an array of transmit antennas at the transmitter based on an outputof the channel measurement, perform a channel measurement for a receiveantenna of the receiver using a signal transmitted from the transmitterto the receiver; apply a reversible transform directly to the channelcoefficients, thus ascertaining channel component coefficientsindicative of an individual weight of respective channel components in atransform domain, wherein the transform domain is an angular domain, andwherein an absolute value of a channel component is indicative of amagnitude in a particular angular direction; select one or more channelcomponents in the transform domain; communicate to the transmitter acontrol signal indicative of one or more preferred channel components ora magnitude of one or more channel component coefficients, in quantizedform, and wherein the control signal comprises at least two fields, afirst field for reporting an inverse discrete fourier transform (IDFT)index corresponding to a peak of the angular domain response and asecond field indicating a value of an amplitude of the angular responseat the peak or a value indicative of a signal-to-interference plus noiseratio (SINR) estimate.
 38. The non-transitory computer-readable storagemedium of claim 37, wherein the reversible transform is linear andorthogonal.
 39. A receiver for a communication network including atleast one transmitter having an array of transmit antennas, the receivercomprising: the receiver including: a channel calculation circuit,configured to be connected with a receive antenna of the receiver andconfigured to perform a measurement of a physical quantity indicative ofa channel quality for the receive antenna on the basis of a receivedsignal and to provide an output signal indicative of the measurementresult in the form of channel coefficients for each of the array oftransmit antennas at the at least one transmitter; a transform circuitconnected with the channel calculation circuit configured to apply areversible transform directly to the channel coefficients, and furtherconfigured to provide at its output channel component coefficientsindicative of an individual weight of respective channel components in atransform domain, wherein the transform domain is an angular domain andwherein an absolute value of a channel component is indicative of amagnitude in a particular angular direction; a selection circuit coupledto the transform circuit configured to select one or more channelcomponents in the transform domain on the basis of the channel componentcoefficients received from the transform circuit; a control circuitconfigured to generate and provide at its output a control signalindicative of one of: one or more preferred channel components resultingfrom the reversible transform applied to angular channel coefficients,the coefficients belonging to the angular domain, wherein the absolutevalue of a coefficient is indicative of a magnitude in a particularangular direction and a magnitude of the one or more channel components,or one preferred channel component resulting from the reversibletransform applied to angular channel coefficients, the coefficientsbelonging to the angular domain wherein the absolute value of acoefficient is indicative of a magnitude in a particular angulardirection and an estimate of a quantity indicative of asignal-to-interference-plus-noise ratio (SINR), wherein the controlsignal comprises at least two fields, a first field for reporting aninverse discrete fourier transform (IDFT) index corresponding to one ofthe preferred channel components and a second field for indicating avalue of the magnitude of said one of the preferred channel componentsor a value indicative of the SINR estimate.
 40. The receiver of claim39, comprising an array of receive antennas.
 41. The receiver of claim40, wherein the receiver is part of a mobile terminal device forwireless communication.
 42. A communication network, comprising: atransmitter comprising an array of transmit antennas for transmission toat least one receiver, and a transmitter control circuit for thetransmitter; a receiver comprising at least one receive antenna, whereinthe receiver is configured to receive a signal transmitted by the arrayof transmit antennas of the transmitter; the receiver including: areceive antenna; a channel calculation circuit configured to: connectwith a receive antenna of the receiver and configured to perform ameasurement of a physical quantity indicative of a channel quality forthe receive antenna on the basis of a received signal transmitted by thearray of transmit antennas of the transmitter and, provide an outputsignal indicative of the measurement result in the form of channelcoefficients for each of the array of transmit antennas at thetransmitter; a transform circuit coupled to the channel calculationcircuit and configured to apply a reversible transform directly to thechannel coefficients, the transform circuit being further configured toprovide at its output channel component coefficients indicative of anindividual weight of respective channel components in a transformdomain; a selection circuit coupled to the transform circuit andconfigured to select one or more channel components in the transformdomain on the basis of the channel component coefficients received fromthe transform unit; and a control circuit configured to generate andprovide at its output a control signal indicative of one of: one or morepreferred channel components resulting from the reversible transformapplied to channel coefficients, the transformed coefficients belongingan angular domain, wherein an absolute value of a coefficient isindicative of a magnitude in a particular angular direction and amagnitude of the one or more channel components, or one preferredchannel component resulting from a reversible transform applied tochannel coefficients, the transformed coefficients belonging to theangular domain wherein the absolute value of a coefficient is indicativeof a magnitude in a particular angular direction and an estimate of aquantity indicative of a signal-to-interference-plus-noise ratio SINR,and the receiver being further configured to construct and provide atits output, beam-pattern control data in the transform domain using thecontrol signal information, wherein the control signal comprises atleast two fields, a first field for reporting an inverse discretefourier transform (IDFT) index corresponding to one of the preferredchannel components and a second field indicating the value of themagnitude of said one of the preferred channel components or a valueindicative of the SINR estimate.
 43. A communication method comprising:receiving a signal from a transmitter; performing a channel measurementfor a receive antenna of a receiver using the transmitted signal;determining channel coefficients for each antenna of the array oftransmit antennas based on an output of the channel measurement;applying a reversible transform from a spatial to an angular domain tothe channel coefficients, to ascertain channel component coefficientsindicative of an individual weight of respective channel components inthe angular domain; selecting one or more channel components in theangular domain; communicating to the transmitter a control signalindicative of one of: one or more preferred channel components resultingfrom the reversible transform applied to channel coefficients, thetransformed coefficients belonging to the angular domain wherein theabsolute value of a coefficient is indicative of a magnitude in aparticular angular direction and a magnitude of the one or more channelcomponents, or one preferred channel component resulting from thereversible transform applied to channel coefficients, the coefficientsbelonging to the angular domain, wherein an absolute value of acoefficient is indicative of a magnitude in a particular angulardirection and an estimate of a quantity indicative of asignal-to-interference-plus-noise ratio SINR, wherein the control signalis configured to be utilized by the transmitter to construct a beampattern in the angular domain, and wherein the control signal comprisesat least two fields, a first field for reporting an inverse discretefourier transform (IDFT) index corresponding to one of the preferredchannel components and a second field for indicating a value of themagnitude of said one of the preferred channel components or a valueindicative of the SINR estimate.
 44. The communication network of claim42, wherein the network is a cellular network.
 45. The method of claim1, wherein communicating the control signal further comprisescommunicating both the one or more preferred channel components, derivedfrom a linear decomposition of a vector formed by the channelcoefficients, and the magnitude of the one or more channel componentsand the one preferred channel component and an estimate of the quantityindicative of the signal-to-interference-plus-noise ratio (SINR). 46.The receiver of claim 23, wherein the control circuit is configured togenerate and provide at its output a control signal indicative of boththe one or more preferred channel components, derived from a lineardecomposition of a vector formed by the channel coefficients, and themagnitude of the one or more channel components and the one preferredchannel component and an estimate of the quantity indicative of thesignal-to-interference-plus-noise ratio (SINR).
 47. The non-transitorycomputer-readable storage medium of claim 37, wherein the control signalis indicative of both the one or more preferred channel components andthe magnitude of one or more channel component coefficients, inquantized form.
 48. The method of claim 43, wherein communicating thecontrol signal further comprises communicating both the one or morepreferred channel components, derived from a linear decomposition of avector formed by the channel coefficients, and the magnitude of the oneor more channel components and the one preferred channel component andan estimate of the quantity indicative of thesignal-to-interference-plus-noise ratio (SINR).